1. Field of the Invention
The present invention relates to flow control devices for solid particulate materials and more specifically to a flow control apparatus for feeding recycled particulate solid material at a controlled flow rate into a fluidized bed combustion vessel from an external heat exchanger. In general, the invention is concerned with a flow control apparatus for use in feeding solid particulate material at selected flow rates over substantial distances from a supply point to an injection point wherein the material is delivered into a vessel for further treatment and handling.
One novel aspect of the invention resides in the formation of a column or head of the material in a standpipe or similar chamber to provide a continuously present seal of material between a supply or source vessel and a receiving or output vessel so that the respective vessels may be maintained at operating pressures independent of one another. The flow rate of material can also be maintained at a selected rate independent of the operating pressure of the vessels involved. Moreover, a flow control device in accordance with the present invention eliminates the need for mechanical valve elements and requires no moving parts to provide the valving flow control action. The flow control device of the present invention is especially well adapted and designed for controlled feeding of recycled particulate solid materials into a fluidized bed combustion vessel from an external heat exchanger. The flow control device is also useful for a wide variety of other applications wherein solid particulate material is involved.
2. Description of the Prior Art
In multi-solid fluidized bed steam generating systems such as disclosed in U.S. Pat. Nos. 4,084,545 and 4,154,581 heating devices known as "L-valves" have been provided for recycling solid particulate material back into a dense fluidized bed in a combustion vessel from a supply source such as a cyclone type separator. A problem associated with such L-valves often occurs when the length of a horizontal feeder conduit or horizontal leg of an L-valve is too great so that the material tends to stagnate or plug up and block off flow through the horizontal leg of the L-valve. Moreover, some types of materials are extremely difficult to move through a lengthy horizontal flow conduit into a remote vessel. Because of these problems, the spacing or distance between the source of recycled fines and the combustion vessel has been limited or in the alternative, mechanical feeders having moving parts and high energy requirements were necessitated. Another problem associated with conventional L-valves is the requirement for a sloping feed conduit rather than a horizontal leg when the distance between the supply of recycled solid material and the combustion vessel become too great to be accommodated with a horizontally extended feeder conduit. In any case, the lack of a suitable flow controllable, feeding device for solid particulate material results in design constraints on the positioning and placement of components in a steam generating system which are undesirable.